Saying someone throws like a girl is not generally acceptable, at least in the age of Title IX and political correctness. Such pronouncements reek of paternalism; certainly we're sophisticated enough to know that gender differences that seem to be innate are really learned, the product of a lifetime of social conditioning.

But the question nevertheless remains: Why, when there are demonstrably better and much less silly looking ways to launch a projectile with one's arm, do girls insist on throwing like girls?

Jerry Thomas, the dean of education at the University of North Texas, has the answer.

A decade or so ago, in New York, a ball came flying over an 18-foot schoolyard fence just as I was passing by. There was no one I could hand it off to, and a gaggle of fifth-graders was waiting for me to toss it back. I had so little faith in my overarm throwing that I had to go underhand. The squeal of brakes was my first indication that the ball had ended up behind me, in the middle of Columbus Avenue. The best I can say about this incident is that nobody got hurt.

There just happens to be a half century of research into the girly-throwing question, which Haspel proceeds to summarize. Mostly, though, she summarizes the work of Jerry Thomas, the dean of education at the University of North Texas.

"The overhand throwing gap, beginning at 4 years of age, is three times the difference of any other motor task, and it just gets bigger across age," Thomas writes in a paper cited by Haspel. "By 18, there's hardly any overlap in the distribution: Nearly every boy by age 15 throws better than the best girl.

Nor is the gap confined to the United States or to societies where boys are taught to throw at a younger age. In Australia, Thomas found that aboriginal girls throw tennis balls at three-quarters the velocity of boys of the same age even though both are taught to hunt early on.

The difference, then, is largely genetic, and not just because males tend to be stronger, as Haspel writes.

The power in an overhand throw -- and in a golf swing, a tennis serve or a baseball swing -- comes from the separate turning of hips and shoulders. The hips rotate forward and the body opens, and then the shoulders snap around. Women tend to rotate their hips and shoulders together, and even expert women throwers don't get the differential that men get. "The one-piece rotation is the biggest difference," says Thomas. "It keeps women from creating speed at the hand." Even when women learn to rotate hips and shoulders separately, they don't do it as fast as men.